When starting a two-stroke engine of a chainsaw, there usually is some mechanism that raises the idling speed of the engine to facilitate the starting. When the engine is started and the user of the chainsaw grips around the rear handle of the saw, he releases the throttle trigger lock and can control the speed of the saw by the throttle trigger. The grip around the rear handle releases the speed raising mechanism, so that the saw now may idle at normal idling speed. However, due to the inertia of the movement of the saw chain, a run-in saw chain may continue running for a few seconds before it stops, and such continued running may be hazardous to the user, both at start and during working with the saw.
To reduce this hazard, U.S. Pat. No. 6,842,987 (Martinsson et al.) proposes a design having a brake triggering arm mounted at the rear handle of the chain saw and connected over a Bowden cable to a pivotable arm, which upon actuation pushes one link of a toggle-link mechanism sideways, so that the force of a kickback spring pulls the brake band tight around the brake drum. Thus, this design gives a manual third braking possibility in addition to a kickback triggered brake and an inertia triggered brake. While this manually operated design, if triggered, will prevent the chain from running at a start of the saw, it will also result in a hard, rapid stop of the chain movement if triggered at high speed, and the accompanying risk of getting a negative kickback, i.e. a downward swing of the tip of the saw chain guide bar. In addition, in course of time, such a brake arrangement will give rise to excessive wear of the brake band.
Further, chainsaws with a so-called kickback brake (also referred to as safety brake or emergency brake) releasable by a handguard have been around for a long time. For these saws, the run of the saw chain is stopped almost instantaneously for safety reasons when a dangerous situation takes place. Such a situation is for instance when a chain tooth at the upper quadrant of the guide bar tip cuts into wood without cutting through it whereby the chain cannot continue moving, and the bar is driven in an upward arc toward the operator; this situation is often referred to as a kickback. When the user's hits the hand guard due to the kickback, the kickback brake is actuated. The kickback may also be released due to inertia of the kickback and/or by other levers than the handguard. U.S. Pat. No. 6,842,987 discloses a design for a kickback brake, hereby incorporated by reference.
There has also been proposed a so-called coasting brake, a chain brake that be actuated independently from the kickback brake. The coasting brake stops the saw chain quickly but without recoil preventing the saw chain to continue to run for a certain time because of the inertia of the saw, after having disconnected the drive. This coasting may cause damages to the saw chain when laying down the device. It may also pose a danger for the user of the saw after the user has released the throttle trigger. Therefore the actuation of the coasting brake is generally coupled with the throttle trigger or with the trigger lock. This means that, when actuating the throttle trigger or the trigger lock, the coasting brake is disengaged so that the saw chain can rotate and when releasing the throttle trigger, the brake is immediately actuated and the run of the chain is stopped.
U.S. Pat. No. 4,683,660, U.S. Pat. No. 5,813,123 and U.S. Pat. No. 6,493,948 belong to a first category where a brake band freely wound around a brake drum rotating with the drive sprocket around which the chain is wound. The coasting brake and the kickback brake are both arranged to pull at a same first free end of the brake band while the second end is fixedly secured to the engine housing of the chainsaw.
U.S. Pat. No. 5,915,795 and U.S. Pat. No. 7,200,941 belong to a second category where a brake band freely wound around a brake drum rotating with the drive sprocket around which the chain is wound, but where the kick back brake is arranged to pull at a first end of the brake band and the coasting brake is arranged to pull at the opposite second end of the brake band.
For both the first and second category a problem is that the wear of the brake band may be considerable during extensive use. Also the brake band may be smoothened during extensive use which may prolong the stopping times during a kickback, which of course is undesirable.
U.S. Pat. No. 5,791,057 shows an electric chainsaw with a kickback brake comprising a brake band freely wound around a brake drum rotating with the drive sprocket around which the chain is wound and a coasting brake comprising an shoe brake which can be engaged with periphery of the brake drum. Also U.S. Pat. No. 5,480,009 shows an electric chainsaw with a kickback brake comprising a brake band freely wound around a brake drum rotating with the drive sprocket around which the chain is wound, and with a coasting brake comprising an interior shoe brake positioned within the brake drum. However these brake arrangements may be too weak for gas chainsaws.